Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/54—Nitrogen compounds
  • B01D53/56—Nitrogen oxides
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B21/00—Nitrogen; Compounds thereof
  • C01B21/20—Nitrogen oxides; Oxyacids of nitrogen; Salts thereof
  • C01B21/24—Nitric oxide (NO)
  • C01B21/26—Preparation by catalytic or non-catalytic oxidation of ammonia

Definitions

• the present invention relates to a process for reducing the concentration of nitrogen oxides in the effluent gases from manufacturing plants.
• Waste nitrogen oxides are released into the atmosphere both in the tail gases from plants manufacturing nitric acid and also in the residual gases produced by various plants using nitric acid as a reagent.
• anti-pollution legislation characteristically sets limits to the weight of nitrogen oxides which may be released per ton of production, and generally excludes resorting to the simple palliative of diluting the exhaust gas with air.
• N 2 O 3 shall be understood to designate the equilibrium mixture of NO + NO 2 ⁇ N 2 O 3 resulting from the partial decomposition of one mol of N 2 O 3 .
• N 2 O 4 shall be understood to designate the equilibrium mixture N 2 O 4 ⁇ 2NO 2 resulting from the partial decomposition of one mol of N 2 O 4 ; or conversely NO 2 represents said equilibrium mixture resulting from the partial dimerization of one mol of NO 2 .
• nitrogen oxide contaminants can be formed by the reaction of nitric acid with reducing agents.
• the products of such chemical reduction are sometimes limited to NO 2 (and N 2 O 4 ) but the reduction may proceed further to NO.
• the effluents of these plants therefore have compositions similar to those found in nitric acid manufacture and can be subjected to the same treatments.
• alkaline absorption which has the advantage of not generating more nitric oxide NO but also the disadvantage of not capturing any free NO.
• the reactions taking place in alkaline absorption are:
• the Na 2 O which can be introduced as NaOH or Na 2 CO 3 takes care of only NO 2 or its N 2 O 3 compound with NO and permits any excess NO to escape, being inert to the alkali but capable of later reacting with atmospheric oxygen to form NO 2 .
• the NO present in the gas to be treated usually is so dilute that its oxidation is very slow and the molar conversion of NO into NO 2 is generally much less than the 50 percent required for complete absorption. Furthermore, the solution of nitrites and nitrates produced by this procedure cannot be released into natural waters but they are too dilute for recovery as commercially usable salts to be practical.
• this invention comprises two steps whereby (1) the effluent gases are oxidized to convert the nitrogen oxides substantially to a mixture of NO 2 and N 2 O 3 and (2) the thus oxidized gases are passed through an aqueous solution containing an amount of hydrogen peroxide maintained at a level just sufficient to oxidize the NO 2 and N 2 O 3 to HNO 3 .
• H 2 O 2 does not oxidize nitric oxide NO to any appreciable extent when pure nitric oxide gas or nitric oxide diluted by nitrogen, argon, carbon dioxide or carbon monoxide is passed through an aqueous solution of H 2 O 2 .
• NO is first oxidized by a suitable agent such as oxygen, ozone or nitric acid, to a mixture of N 2 O 3 and N 2 O 4 , these gases are easily absorbed in H 2 O 2 solution according to the overall reactions:
• HNO 3 hydrogen peroxide
• the upper limit of thus permissible HNO 3 concentration is generally around 50 percent by weight but it depends also on temperature and hydrogen peroxide concentration.
• the present inventors have found that in the course of absorbing N 2 O 4 and N 2 O 3 conditions are developed which cause a high rate of decomposition of H 2 O 2 into H 2 O and O 2 .
• the process for reducing the nitrogen oxide content of effluent gases according to the present invention is characterized by the stepwise accomplishment of (a) an oxidation of NO to N 2 O 3 or NO 2 in the gas phase, followed by (b) an absorption in aqueous medium in presence of hydrogen peroxide carried out in such a manner that the quantity of H 2 O 2 introduced is just sufficient to oxidize N 2 O 3 or NO 2 to HNO 3 with substantial avoidance of excess H 2 O 2 .
• the oxidation step is carried out by HNO 3 but any oxidation agent capable of oxidizing NO can be utilized therefor.
• This process can be used to treat any residual gas containing nitrogen oxides and is of particular interest as a manner of reducing the concentration of nitrogen oxides in the effluents from nitric acid manufacturing plants, as well as for the treatment of residual gases in chemical plants using nitric acid, such as those for passifying stainless steel and for nitration of organic compounds.
• nitric acid When nitric acid is used to effect the first oxidation step, this can be carried out by mixing the effluent gases with gaseous nitric acid in any suitable manner at a pressure from 1 to 20 bars, a temperature from -10° C to +40° C.
• the reaction can be brought about, for example, by bubbling the effluent gas through an aqueous solution of nitric acid or by spraying the acid into the gas. It has been established that it is most suitable to use an installation wherein perfect liquid-gas equilibrium is maintained.
• nitric acid concentration of nitric acid is dependent on the composition of the treated gas and the temperature. It is generally desired to use nitric acid substantially free of nitrogen oxides in order to lessen the consumption of hydrogen peroxide.
• the concentration of nitrogen oxides in the treated gas is generally between about 1,000 and 2,000 cm 3 /m 3 which necessitates using HNO 3 of concentration 20-70% by weight at ambient temperature. In the case of other residual gases, HNO 3 concentration can vary from 20-85 percent.
• the gases are treated in liquid medium by passage through an aqueous solution receiving continuously H 2 O 2 in quantity such that the quantity of H 2 O 2 introduced is substantially that which is necessary to oxidize the N 2 O 4 and N 2 O 3 while avoiding an excess of H 2 O 2 .
• the treatment is generally effectuated at ambient temperature under a pressure of 1 to 20 bars on the gases which exit from the nitric acid oxidation at a temperature of -10° C to + 40° C.
• the reaction is very rapid; therefore any equipment can be used which accomplishes substantially complete contact between gas and H 2 O 2 solution.
• Inventors have established in particular that an installation corresponding to one theoretical plate serves particularly well.
• the process for reducing the nitrogen oxide content of gaseous effluents according to the instant invention is particularly applicable to reducing the nitrogen oxide content of effluents from plants manufacturing nitric acid having absorption columns in stages. It suffices to subject one or several of such stages to the oxidation of the effluents by nitric acid and one or several other stages to the H 2 O 2 treatments.
• a particular advantage of the instant method is its flexibility in that the quantity of H 2 O 2 used can be adjusted to correspond to any desired degree of purification and to correspond to the changing variations during the course of production.
• the method is particularly adaptable to automatic control of the H 2 O 2 used in proportion to the changing concentration of nitrogen oxides in the residual gases.
• the aqueous phase at the start is plain water.
• the nitrogen oxide is pure NO 2 .
• there is formed only 253 mg. of HNO 3 corresponding to 49% of the acid stoichiometrically equivalent to 2,000 cc/m 3 of NO 2 according to equation (2) above.
• the residual nitrogen oxides are totally absorbed in the sodium hydroxide, consisting therefore of a mixture of NO 2 and N 2 O 3 and the salts formed are a mixture of NaNO 2 and NaNO 3 very rich in nitrite.
• the aqueous phase is an aqueous solution of 40 grams of H 2 O 2 per liter.
• the nitrogen oxide is pure NO 2 .
• the amount of HNO 3 formed is 633 mg. and 0.42 grams of H 2 O 2 have disappeared.
• the final concentration of H 2 O 2 is 31.6 grams/liter.
• the 633 mg. of HNO 3 corresponds to 121% of the acid which would theoretically result from the reaction (2) upon the 2,000 cc/m 3 of NO 2 .
• the residual nitrogen oxides are totally absorbed in the sodium hydroxide, forming a 50/50 molar ratio of NaNO 3 and NaNO 2 , corresponding to the effluent nitrogen oxide gas being entirely NO 2 .
• the consumption of H 2 O 2 is 1.23 mols of H 2 O 2 per mol of HNO 3 formed.
• the aqueous phase is an aqueous solution of 40 grams of H 2 O 2 per liter.
• the nitrogen oxide is pure NO.
• the amount of HNO 3 formed is 24.4 mg. and 0.05 grams of H 2 O 2 have disappeared, the final concentration being 39 grams/liter.
• the 24.4 mg. of HNO 3 corresponds to 3% conversion of the NO supplied at 2,000 cc/m 3 .
• the residual nitrogen oxides are not absorbed in the sodium hydroxide, only traces of NaNO 2 being formed.
• the consumption of H 2 O 2 is 3.8 mols of H 2 O 2 per mol of HNO 3 formed.
• the aqueous phase is an aqueous solution of 40 grams/liter of H 2 O 2 and the nitrogen oxide is a mixture of equal parts of NO and NO 2 corresponding to N 2 O 3 .
• the amount of HNO 3 formed is 470 mg. and 0.56 grams of H 2 O 2 have disappeared, the final concentration being 28.8 grams/liter.
• HNO 3 The 470 mg. of HNO 3 corresponds to 180% of the acid which would be expected to form from 2,000 cc of N 2 O 3 per cubic meter, according to the reaction (1) above.
• the nitrogen oxide residuals are totally absorbed in the sodium hydroxide and form NaNO 2 with a small amount of NaNO 3 .
• the consumption of H 2 O 2 is 2.20 mols per mol of HNO 3 formed.
• the aqueous phase at the start is plain water, and the nitrogen oxide is pure NO 2 .
• the nitrogen oxide is pure NO 2 .
• the amount of HNO 3 formed is 460 mg. and the total disappearance of introduced H 2 O 2 is 147.
• the 460 mg. of HNO 3 corresponds to 87.5% of the stoichiometric amount corresponding to the 2,000 cc/m 3 of NO 2 .
• the corresponding yield in Example (1.1) is only 49 percent.
• the nitrogen oxide residuals are totally absorbed in the sodium hydroxide and form a 50/50 molar mixture of NaNO 3 and NaNO 2 .
• the consumption of H 2 O 2 is only 0.59 mols per mol of HNO 3 formed as compared to 0.50 mols calculated theoretically from the reaction
• the last absorption column of an installation for the nitric acid production of type 165/T/J having 16 plates operating under a pressure of 4 bars absolute receives per hour 22000 cubic meter of gas containing 0.15 volume percent of nitrogen oxides and releases a residual gas containing 0.07% volume percent of nitrogen oxides.
• the residual gases contain between 0.017 and 0.018 percent of nitrogen oxides and this is confirmed by analysis of samples of the 10 highest plates.
• H 2 O 2 under these conditions shows that it is possible to reduce the residual nitrous oxide concentration to 200 cc per cubic meter by consuming 6.9 Kg. of 100% H 2 O 2 per ton of nitric acid leaving the installation.
• Example 2 During a summer period when the exterior temperature can reach 25° C the last column of the same installation as in Example 2 receives 0.35 volume percent of nitrogen oxides and releases at the rate of 22000 cm/h a residual gas containing 0.15 volume percent.
• Plates 10 and 15 again have a considerable concentration of H 2 O 2 , namely 2 and 6 grams/liter respectively, showing that the utilization of H 2 O 2 has been incomplete.
• This example describes an industrial-scale operation in a column carrying out successively the oxidation of nitric oxide, NO, by nitric acid in the vapor phase, followed by the absorption of the resulting oxidized gases by hydrogen peroxide in aqueous medium.
• the column used in this example contains 16 plates and is the last column of a unit producing 165 tons of nitric acid per day and operating under an absolute pressure of 4 bars. During a summer period when the exterior temperature can reach 25°C, this column receives gases containing 0.35% of nitrogen oxides. Without the modification of this example, the gas released at the rate of 22000 cubic meters per hour from this column contains 0.15% of nitrogen oxides.
• the plate situated at the top of the column (plate 16) receives, as in the preceding examples, the remaining water, flowing at about 750 liters/hr., which circulates through three plates before reaching those which receive hydrogen peroxide.
• the consumption of hydrogen peroxide corresponds to 5.1 Kg. of 100% H 2 O 2 per ton of nitric acid produced.

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• Chemical & Material Sciences (AREA)
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Citations (4)

• 1973
  • 1973-07-30 FR FR7327774A patent/FR2239279B1/fr not_active Expired
• 1974
  • 1974-06-17 BE BE145495A patent/BE816419A/xx not_active IP Right Cessation
  • 1974-07-12 IE IE1479/74A patent/IE39606B1/xx unknown
  • 1974-07-24 US US05/491,455 patent/US3991167A/en not_active Expired - Lifetime
  • 1974-07-25 SE SE7409668A patent/SE413228B/xx not_active IP Right Cessation
  • 1974-07-26 NO NO742733A patent/NO139633C/no unknown
  • 1974-07-27 DE DE2436363A patent/DE2436363B2/de active Pending
  • 1974-07-29 NL NL7410187A patent/NL7410187A/xx not_active Application Discontinuation
  • 1974-07-29 CA CA205,885A patent/CA1025637A/fr not_active Expired
  • 1974-07-29 IT IT69412/74A patent/IT1016726B/it active
  • 1974-07-29 BR BR6204/74A patent/BR7406204D0/pt unknown
  • 1974-07-29 CH CH1043374A patent/CH587677A5/xx not_active IP Right Cessation
  • 1974-07-29 DD DD180181A patent/DD112720A5/xx unknown
  • 1974-07-29 GB GB3345174A patent/GB1473146A/en not_active Expired
  • 1974-07-30 JP JP8667574A patent/JPS5626459B2/ja not_active Expired
  • 1974-07-30 AT AT624174A patent/AT331264B/de not_active IP Right Cessation
  • 1974-07-30 ES ES428776A patent/ES428776A1/es not_active Expired
  • 1974-07-30 ZA ZA00744857A patent/ZA744857B/xx unknown

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